1. Field of the Invention
The present invention relates to a secondary battery and its method of manufacture, and more particularly, the present invention relates to a secondary battery, and its method of manufacture, which can absorb external shocks by forming wrinkles between a rupture section and a periphery of a depression formed in a cap plate or a can, and thus prevent damage of the depression due causes other than the internal gas pressure of the battery.
2. Description of the Related Art
Generally, unlike a primary battery which can not be charged, a secondary battery is a battery in which charging and discharging can be performed, and is widely used in state-of-the-art fields, such as cellular phones, notebook computers, camcorders and so on. In particular, since a lithium secondary battery operates at 3.6V, which is three times the operating voltage of a nickel-cadmium battery or a nickel-hydrogen battery which are widely used as a power source for electronic equipment, and since the energy density per unit weight is very high, lithium secondary batteries are being widely used.
Such a lithium secondary battery mainly uses a lithium-system oxide as a positive electrode active material, and an elastic material as a negative electrode active material. Furthermore, the lithium secondary battery can be formed in various shapes, such as a cylindrical shape, a polygonal shape, and a pouch shape.
Of these shapes, a polygonal shaped secondary battery includes an electrode assembly, a can into which the electrode assembly is contained, and a cap assembly connected to the can.
The electrode assembly is composed of a negative electrode, an positive electrode and a separator wound between them, and a negative electrode tab and a positive electrode tab are respectively drawn out from the positive electrode and the negative electrode.
The can is a metal container having an approximately rectangular parallelepiped shape for a polygonal shaped secondary battery, and is formed by a processing method such as a deep drawing and so on.
The cap assembly is composed of a cap plate connected to the upper side of a can, an electrode terminal which is formed via a terminal through hole, and on the external side of which a gasket is arranged for insulation from the cap plate, an insulation plate arranged on the lower side of the cap plate, and a terminal plate arranged on the lower side of the insulation plate for allowing the current to flow to the electrode terminal.
The electrode assembly is electrically connected to the electrode terminal via the negative electrode tab and the terminal plate, and the positive electrode is electrically connected to the cap plate or the can via the positive electrode tab.
On the other hand, a depression is formed on one side of the cap plate. Since this depression is formed to be thinner than other portions around it, it is preferentially ruptured when the internal pressure of a battery increases due to overcharging, and thus, the gas is discharged, thereby protecting the battery. Such a depression is classified as a clad depression and a press depression according to its formation method.
However, a conventional depression has a following problem.
Generally, the thickness of a cap plate is 0.8-1.0 mm, the thickness of a cap plate at the site where the depression is formed is 70 nm, and the thickness of a cross-section where damage is expected in the depression when a problem occurs in a battery is merely 20-30 nm.
As described above, the depression is originally ruptured due to the internal pressure of a battery, and thus performs a function to protect the battery. However, since the thickness of the rupture section in the depression is merely several-tens of nano meters, a crack can occur in the rupture section due to even small external shocks or the rupture section can be ruptured. That is, the depression section can be ruptured, and the reliability of the battery reduced.